The avalanche photodiode type light detector is used frequently as the sensor in optical receiver systems. With the ever increasing demand upon the performance of military detection and sensing systems, e.g., extended range laser trackers, there is a concomitant rise in the performance needed from the avalanche photodiode, in terms of sensitivity, frequency response, optical band filtering and signal-to-noise ratio.
The ultimate in receiver system performance is attained when a low noise amplification system is combined with a photodiode having extremely low capacitance, high quantum efficiency, low leakage, low series resistance and sufficient avalanche gain to raise the signal level above the noise of the preamplifier. In addition, the fabrication materials and configuration must be such that the detector exhibits these characteristics at the proper wavelength, and in most cases selectively excludes unwanted energy at other wavelengths.
The design and fabrication of general avalanche photodiodes is well known in the art, as is evidenced by the issuance of a number of patents encompassing this class of devices, such as U.S. Pat. Nos. 3,436,613 and 3,534,231. Though heterojunction avalanche photodiodes configured and fabricated as taught in the prior art are able to satisfy many of the performance goals, they are deficient in two aspects, the high frequency avalanche gain attainable and gain uniformity. These inadequacies occur because lattice mismatches and growth nucleation non-uniformities introduced into the junction area during the fabrication process lead to local areas of ionizable defects or reduced breakdown voltages.